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RF Load Failure Detection Circuit

Publishing Venue

Motorola

Related People

Authors:

Rodney Hagen

Abstract

RF power combining loads can be large and costly. Their purpose in a coherent combining system is to provide isolation to the other circuits “to be combined”. They also must perform this isolation function in the event one or more of the circuits “to be combined” fails. If the circuits “to be combined” are active devices (e.g. amplifiers), the load must large enough to dissipate up to one half of the RF output power from the remaining amplifier, in the event of an amplifier failure. The reason is, if the load fails, the other amplifier(s) can oscillate (due to unfavorable impedance’s) causing a FCC violation. Figure 1 shows the block diagram of a typical parallel staged amplifier.

Copyright

Motorola, Inc. 10/02/02

Country

United States

Language

English (United States)

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RF Load Failure Detection Circuit

By Rodney Hagen

Problem:

RF power combining loads can be large and costly.
Their purpose in a coherent combining system is to provide isolation to the
other circuits “to be combined”. They also must perform this isolation function
in the event one or more of the circuits “to be combined” fails. If the
circuits “to be combined” are active devices (e.g. amplifiers), the load must
large enough to dissipate up to one half of the RF output power from the
remaining amplifier, in the event of an amplifier failure.� The reason is, if the load fails, the other
amplifier(s) can oscillate (due to unfavorable impedance’s) causing a FCC
violation.� � Figure 1 shows the block
diagram of a typical parallel staged amplifier.

One could make the
load larger, but that would cause the product to be undesirably larger. Also,
the larger loads can be up to 5 or 6 times more expensive than smaller loads
that work well in non-fail conditions.� �
Moreover, in some cases the failure of a singular device necessitates
the eventual shut down of an entire Base Radio due to reduced amplifier
headroom.� In essence, the idea of
paying for an expensive load to keep the amplifier stable (not oscillating) for
the relatively small time before shut down occurs is not desirable.

In addition, large surface mountable loads currently do
not work well on newer amplifier technologies using the more traditional
fiberglass or Teflon printed circuit boards (PCB). This is because the loads
can not easily get rid of the heat on the newer PCBs due to the PCB’s higher
thermal resistance.� Furthermore, when
larger loads are used to overcome the poorer thermal conduction of PCBs, they
tend to aggravate the usual coefficient of thermal expansion (CTE) mismatches
that exist between the load and the PCB.�
This in itself can cause premature product failure.

That leaves the flanged (or bolted down loads) which do
not usually have CTE issues, or they are at least minimized.� However, they are even larger and more
costly than the surface mountable units.

Figure 1

Solution:

A parallel staged amplifier that uses the “RF Load
Failure Detection Circuit” (Figure 2) allows a designer to use a low cost load
that will perform its job while all amplifiers are operating in a non-failed state.� In the event of an amplifier failure, the RF
combiner’s isolation load will open. This load failure will be detected by the
“RF Load Failure Detection Circuit”, whi...